Most common electrical motors have a rotational output, but in many instances the structure to be driven requires a linear motion. There are a number of approaches to mechanically converting the rotational motor output to a linear motion. One such approach is a leadscrew mechanical drive in which the motor rotationally drives a threaded leadscrew. A leadscrew follower structure driven by the leadscrew engages the leadscrew and moves parallel to the axis of the leadscrew to convert the rotational motion to linear motion parallel to the axis of the leadscrew. The leadscrew follower structure typically includes a follower threadably engaged to the leadscrew or a recirculating ball mechanism, although other types of leadscrew followers have been proposed.
The conventional leadscrew mechanical drive works well in many situations, but has limitations in others. If the linear motion is to be very slow, the basic leadscrew mechanical drive must be modified in one of several ways. The leadscrew must have a small thread pitch, the leadscrew must be driven through a speed-reducing gearbox placed between the motor and the leadscrew, and/or special motor controls such as a high-resolution rotary optical encoder must be used to measure and control the motor output in a feedback manner. Additionally, care must be taken to minimize play and backlash in the leadscrew mechanical drive. Some of these modifications have physical limitations that limit the ability to achieve the desired slow linear movement. For example, the reduction in thread pitch is limited by the physical width between the turns of the thread that may be achieved by machining or other thread-producing technique. The efficiency of speed-reducing gearboxes falls with increasing reduction ratio, requiring a larger motor for high reduction ratios. The ability to reduce the rotational output speed of the motor is limited by the structure of the motor windings, so that it may be necessary to use a special low-speed motor. In any event, these modifications lead to an expensive, bulky, and/or heavy leadscrew mechanical drive that may not be suitable for the desired applications.
There is therefore a need for an improved approach to the mechanical conversion of rotary motion to linear motion, particularly for applications in which the required linear motion is very slow. The present invention fulfills this need, and further provides related advantages.